The shafts of outrunners are usually easily removed, So you could machine the sprocket's hub much shorter and get it TIG welded to the shaft to save space and make sure it never shifts. Set screws are always a bit dodgy - they come undone at the worst time.

After looking at your CAD, is there any reason why you couldn't save space by cutting a hole or a pocket in the top plate, just above the sprocket hubs? It looks like you already did that under the back of the motor - is the front of the motor in a pocket in the middle plate?_________________Australian 2015 Featherweight champion
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Wed Jan 18, 2017 6:41 am

chunkulator

Joined: 27 Jul 2016
Posts: 211

If I don't shorten the sprockets, the "pocket" would need to go all the way through the top plate exposing the shaft at the top (which may not be the end of the world). But, yes, if I can shorten the sprockets then a pocket could ease the pinch by a few mm.

I also tried the Scorpion motor in CAD on my way to work this morning and it makes the whole thing much easier. I can fit everything without messing with the sprockets but it costs me an extra $100. I'll post some updates CAD screenshots later.

I was hoping to keep the sprockets interchangeable until I find the optimal gear ratio. But once I know that I could totally weld the (hubless) sprockets on.

Wed Jan 18, 2017 12:10 pm

maddox

Joined: 21 Dec 2006
Posts: 758
Location: Belgium

Or have the shaft and gear machined from a solid piece of steel.

Wed Jan 18, 2017 12:37 pm

NickExperienced Roboteer

Joined: 16 Jun 2004
Posts: 11745
Location: Sydney, NSW

Its definitely not the end of the world to have a couple of small(ish) holes in the top. Only axe and crusher bots have easy access to the top of other bots and we don't have many (or any!) of those active at the moment.

With that scorpion and similar sized motors, the internal shaft is often 8mm and steps down to 6mm to fit a propeller hub. Its handy to buy a short length of 8mm silver steel to replace the original shaft and that will make welding the sprocket easier & stronger._________________Australian 2015 Featherweight champion
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Wed Jan 18, 2017 12:40 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Good to know. That means I can use the 8mm ID sprockets i already have instead of ordering new 6mm ones too.

Does it matter if I use ground 4140 instead of silver steel for the shaft? I already have a bunch of that.

4140 requires special treatment when welding I've heard, but I'm not sure about silver steel.

Wed Jan 18, 2017 4:51 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Also a good suggestion Maddox. I suspect i may be able to cut a sprocket on the end of some 30mm round bar with my CNC machine if I can find the centre and then turn down the shaft. The welding route sounds more achievable with garage technology though.

Wed Jan 18, 2017 4:54 pm

NickExperienced Roboteer

Joined: 16 Jun 2004
Posts: 11745
Location: Sydney, NSW

4140 should be fine - I only suggested silver steel as it is easy to get precision ground 8mm rods. 4140 is probably easier to weld to a sprocket too._________________Australian 2015 Featherweight champion
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Wed Jan 18, 2017 5:00 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Of course, the other solution would be a mount a tyre directly on the outside of the motor can if it had good bearings, which it probably doesn't ...

Wed Jan 18, 2017 5:18 pm

NickExperienced Roboteer

Joined: 16 Jun 2004
Posts: 11745
Location: Sydney, NSW

Direct drive is a sure way to kill the motor! Even if the small bearings don't crap out, the motor can tear out the mounting screws or separate the magnet ring from the endbell_________________Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Wed Jan 18, 2017 5:31 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Well that's clearly a bad idea then.

Wed Jan 18, 2017 5:43 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Turns out the Turnigy T600 motor is 6mm all the way through. I just pulled one apart. I can confirm that the shaft is removable though.

Oh well, I guess I can weld a 6mm sprocket to it.

Wed Jan 18, 2017 7:08 pm

NickExperienced Roboteer

Joined: 16 Jun 2004
Posts: 11745
Location: Sydney, NSW

Luck of the draw Not that I am a Scorpion motor salesman, but on some motors you can see a taper on the shaft right at the front of the motor. If the specs say its a 6mm shaft and you see a taper, its a good bet that the shaft inside the motor is at least 8mm._________________Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Wed Jan 18, 2017 7:19 pm

chunkulator

Joined: 27 Jul 2016
Posts: 211

Anyone got a good source for 15mm ID disc springs? Blackwoods, Konnect/Coventry, element14, eBay and even McMaster Carr seem to be no help. Some of them stock 16mm Bellevilles, which are much thicker than disc springs and don't seem to offer any give unless you're talking tonnes of earthmover or something. The only 16mm ones stocked at Blackwoods are made from 3mm thick steel and have no give whatsoever, for example.